Scramjet with stamina ready for hypersonic test

By Rachel Courtland

LATER this month, thousands of square kilometres of airspace above the Pacific Ocean will be cleared to make way for a skinny, shark-nosed aircraft called the X-51.

The 4-metre-long pilotless prototype will drop from beneath the wing of a bomber and attempt to become the first supersonic combustion ramjet – or scramjet – to power through the air at hypersonic speeds for minutes, not seconds.

A scramjet compresses air enough to ignite fuel, which drives exhaust out of the back of the engine to provide thrust. It uses the speed of the aircraft to compress the air and therefore is designed to work only at hypersonic speeds – above about five times the speed of sound.

A few experimental craft have flown successfully, reaching speeds as high as Mach 10, but the extreme heat generated by flying at those speeds has curtailed the length of time their engines have operated.

“No one has successfully flown a vehicle of this nature for more than a few seconds,” says Joe Vogel, X-51 programme manager at Boeing, one of the firms collaborating with US military agencies to build the plane. “Our goal is about 300 seconds of powered flight.”

No one has successfully flown a vehicle of this nature for more than a few seconds

At the test’s top speed of Mach 6, the X-51’s nose will reach 1480 °C, says Vogel. To handle the heat, fuel is piped through tubes around the surface of the engine. Not only does this draw off heat to stop the engine melting, it also helps warm the fuel enough to be ignited.

The US air force hopes to conduct ...

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